Field of Invention
The present invention relates to liquid crystal displays, and particularly to how gray level voltages are generated for active matrix liquid crystal displays to adjust or compensate for brightness and color variations, whether instructed or caused by operating conditions.
A. Problems in the Art
1. Basic LCD structure and operation PA1 2. Segmented LCDs PA1 3. Matrix LCDs PA1 4. Color LCDs PA1 5. Advantage of LCDs PA1 6. Deficiencies and problems with existing LCD's PA1 7. Example of deficiencies and problems with aircraft instrumentation
There are several types of liquid crystal displays (LCDs). The general concept of operation of liquid crystal displays is the same for all types. A liquid crystal material is placed in a sealed but light transmissive chamber. Light-transmissive electrodes are placed above and below the liquid crystal material. In one type of LCD utilizing what are called twisted nematic liquid crystals, when a sufficient electric potential is applied between the electrodes, the liquid crystal molecules change their alignment. The change in alignment alters the polarization state of light through the liquid crystal material. The chamber or cell essentially acts as a light shutter or valve. It lets either a maximum or minimum of light through; or some intermediate level.
Therefore, by putting the liquid crystal chambers on top of a polarized back-lighting source, and locating a polarizer above the LC chamber, the top of the liquid crystal chamber will look either black (dim) or white (bright) depending on the alignment of the material (whether the valve is "closed" or "open"). By using a collection of these chambers, such things as letters, numbers, or graphics can be formed by applying appropriate voltage potential across certain chambers which instruct the appropriate cells to pass or block light, to in turn form the appropriate visual pattern.
Many watches utilize liquid crystal displays. Some advantages of LCD's for watches include the small amount of space required by the display and the circuitry driving it, as well as the low power consumption generally required. Watches generally use what is called segmented LCD structure. Seven segments or chambers of liquid crystal material can be arranged into a template or pattern which can form any numeral (and virtually any letter). Each segment is then controlled to simply turn "on" or "off"; that is, transmit maximum light (appears brighter or white) or not (appears dim or black). The appropriate segments are turned "black" to form the desired numeral if a "normally white" background is used. It is to be understood that depending on design choice, the background of liquid display can either be normally black or normally white.
The advantage of a segmented display for watches is the simplicity of the display and associated structure. Only a few segments must be operated to form each number. Also, the segments usually must be driven to be fully on or fully off. This simplifies the electrical structure needed to drive and control the LCD.
For more complex displays, a matrix LCD structure is normally utilized. A large number of very small independent regions of liquid crystal material are positioned in a plane. Each of these regions is generally called a picture element or pixel. These pixels are arranged in rows and columns forming a matrix. Corresponding numbers of column and row electrodes are correlated with the rows and columns of pixels. An electric potential can therefore be applied to any pixel by the selection of appropriate row and column electrodes and a desired graphic can then be generated.
There are different types of matrix LCD structures. Some are considered "active matrix" LCD's; others are called "passive matrix" LCD's. The present invention is related primarily to active matrix LCD's and therefore this discussion shall concentrate primarily on that form of LCD.
An active matrix LCD operates in an analogous way to other video-type displays. The individual pixels comprise dots or small portions of the overall picture or graphic to be displayed. A graphic control therefore produces the appropriate instructions to "drive" the matrix of pixels to appropriately reconstruct the image desired. In other words, the control circuitry must send appropriate voltages to appropriate pixels at appropriate times to form the correct image.
This is a complex process. If the LCD is used as a television screen, for example, the image constantly changes. Similarly to conventional cathode ray tube (CRT) televisions which scan an electron beam across successive rows of pixels, the matrix of LCD pixels is generally scanned at many times per second (for example, 20 times per second) by parallely charging pixels in successive rows of the display to continually update the composite picture. In some displays the graphics are created by turning selected pixels "black" and leaving the others "white". By varying the density of black pixels, shades of gray can be somewhat simulated. The driving circuitry only has to turn pixels fully "on" or fully "off". For many applications it is also desirable that each pixel not only must be able to turn completely "on" and "off" (that is the states of "black" or "white"); it must also be able to have varying degrees of transmissibility between totally black and totally white. An individual pixel can then be black, white, or different shades of gray in-between.
This is called the gray scale ability of a pixel. To provide gray scale, the control circuitry must be able to supply varying levels of voltage potential to each pixel. The voltage is correlated to each succeeding darker shade of gray. As is well known in the art, gray scale voltages for a particular LCD are determined from the type of liquid crystal material used, the spacing of the electrodes, and other factors. In other words, it can be determined what different levels of voltage will cause what different transmissibilities of the pixel to occur. Once this is known, the control circuitry can issue appropriate instructions to request the appropriate gray levels to replicate the input signal (for example the video signal). The circuitry then must also generate the actual voltages correlated to the instructed gray scale levels.
It can therefore be understood that if the pixel size is sufficiently small, and the size of the matrix is sufficiently large complex graphics with high resolution can be displayed.
Television pictures, and even color television, can be replicated on matrix LCDs; as can virtually any graphic. Color is created by placing blue, green, and red filters over selected pixels, generally in a repeating pattern across the display (for example, delta-triad, diagonal mosaic, or quad patterns). The graphics control for the LCD would then instruct the correct combination of activation of red, blue, and/or green pixels, in addition to the correct gray scale for each pixel behind the red, green, or blue filters. Although the LCD-actually displays a matrix of discrete red, blue, and green filtered pixels driven to varying transmissities (gray levels), the human eye would then "average" closely spaced positions of the screen. Instead of seeing individually lighted red, blue or green pixels, the viewer would perceive regions of colors along the whole range of the color spectrum. For example, the human eye will average a green and red side-by-side pixel combination to be the color yellow, if the intensities or brightness of the green and red LCDs are essentially the same. This concept is well known in the art.
The use of LCD's is becoming more wide spread for at least the following reasons. As mentioned above, they require relatively low power consumption, at least as compared to cathode ray tubes (CRT's). Liquid crystal technology is becoming more advanced, which in turn allows better pixel performance and better resolution. A substantial benefit is the fact that the technology of LCD's allows the dimensional depth of a LCD display and its associated circuitry to be much less than CRT's. Moreover, the weight of a LCD is much less than a CRT of comparable size. Still further, from a safety standpoint, most LCD's eliminate the risk of the presence of high voltage (up to several thousand volts with CRT's) and other problems associated with CRT technology.
While the advantages of LCD's sound encouraging, several problems do exist with LCD's which are related to their physical makeup. As discussed above, a liquid crystal pixel can be driven to white, black, or an intermediate gray scale level by altering the transmissibility of the light through the pixel by using correlated gray scale drive voltages. Thus, the optical characteristics of LCD's are such that if a viewer is looking directly normal (on-axis) to a pixel or a collection of pixels, and the pixel or a collection of pixels is appropriately driven, the image created by the pixel or collection of pixels will have uniform intended contrast with the background (or alternatively an uniform brightness). However, if a viewer's line of sight moves substantially off-axis (for example more than a few degrees), the optical characteristics change with respect to that location even though the driving voltage remains the same.
In simple terms, a change in viewing angle can significantly change how well the images can be seen from that angle. An easy example of this phenomenon is to take a LCD watch and view the display straight on (on-axis). Because the display is small and flat, all numbers or characters and all parts of the screen look fairly clear and consistently distinct across the display. In other words, the numbers are "bright" (dark black on a white background, or white on a dark black background). However, if you angularly tilt the watch display to a severe angle with respect to your eyes, the characters on the display become much less distinct and can virtually disappear (i.e., the brightness contrast between numbers and background degrades). This can occur at both vertical or horizontal off-axis viewing angles. Vertical angles tend to cause more problems however.
A significant problem therefore exists in the lack of consistency of sharpness, clarity, or distinctness of graphics displayed on a LCD as a function of viewing angle. This may not be very significant with a LCD watch, because the user can easily either maneuver the watch or the user's head to a position generally normal to the display surface and because the watch does not use intermediate gray shades. It can, however, be a substantial and even critical problem in such LCD uses as, for example, control instrumentation where consistent and accurate viewing is essential. As can be appreciated for a large LCD, the viewing angle of the viewer's eyes between the top of the screen and the bottom of the screen can even cause different parts of the screen to vary.
Another problem or concern with LCD's is the fact that performance of the display can vary as a function of temperature of the display. Ambient temperature variations or internal temperature rises due to back-light power dissipation change the panel temperature, which in turn affects the performance of the liquid crystal or the electronic components. The result can be the loss of contrast between pixels which are supposed to be at a gray level distinct from the background. This again relates to what is called "brightness" of the images versus the background.
Still further, a deficiency in the art exists with respect to the ability to selectively vary brightness or contrast of certain portions of the display (foreground vs. back-ground). Normally, if the entire display needs to be dimmed, the art generally utilizes a control to simply change the background illumination to the entire display. However, there are occasions and needs where it would be advantageous to dim or to brighten selected portions of the LCD.
A specific example of how these problems affect real world situations is to consider the utilization of LCD's on instrumentation in aircraft. Currently either analog electro-mechanical instrumentation or CRT displays are generally used for certain primary flight instruments. Many of these instruments require a screen of several inches in diameter. They would also be placed in a control panel anywhere from several inches to several feet away from the pilot's eyes. Some of the instrumentation would be positioned at the copilot's position, but still require viewing by the pilot.
Although the control panel is generally configured to present favorable viewing orientation for the pilot and copilot, the angle of view both horizontally and vertically for all gauges cannot be directly perpendicular or normal (on-axis) to the pilot's eyes for all instrument displays, or at least for all portions of a display. Therefore, the viewing angle problem is significant. It is easily understood how lack of contrast (brightness) on an instrument or a gauge display in an airplane, to the point where the pilot cannot easily comprehend the information on the instrument or gauge, can be a significant problem; and even a dangerous problem.
Moreover, continual operation of this instrumentation over a variety of lengths of time, altitudes, ambient air temperatures, etc., can affect the operational temperature of the LCD's. This in turn can affect the optical performance of a LCD as previously described.
Still further, it is desirable in some of the complex instruments and displays for commercial aircraft that certain portions of the display be independently dimmed or brightened. An example is the ability to bring up a brightened radar display graphic on a portion of the same display that other information is being displayed. It is essential that this radar image also be able to be removed or at least dimmed on command when desired so that the other information can be clear and distinct.
It can therefore easily be seen that substantial problems and deficiencies exist in the LCD art. A real need exists in the art to solve or at least improve the ability to deal with these problems and deficiencies.
It has also become apparent that the viewing angle problems associated with LCDs can materially affect color rendition of LCDs using colored filters over the pixels to reproduce color graphics. This problem is significant because it not only affects color rendition, but could also affect the perception of what type of information is being displayed on the LCD. For example, an airplane gauge or instrument may use color coding for certain symbols or areas of display. Pilot becomes accustomed to perceiving a certain color for a certain condition. If the viewing angle is severe enough, a display portion can actually change colors. This can represent a dangerous situation.
The problem is caused by the same factors causing brightness variations as previously discussed. As off-axis viewing angle increases, brightness of the pixel will change from the brightness when viewed straight on-axis. This changes the perceived gray scale level of the pixel. If the color filter is positioned over the pixel, this will affect the intensity or luminance of the pixel. As is known in the art, colors are generated in LCD displays by clustering red, blue, and green color filtered LCDs in repeating patterns across the display. Different colors can be created in different locations in the screen by turning individual blue, green, red pixels onto different intensities. The human eye averages individual luminance or intensities of each colored pixel and constructs a shade of color according to those different intensities.
For example, to produce a purely red field, all the green and blue pixels are driven completely "black" (to basically turn them black or to block any light transmitting through the pixel). All the red LCDs are driven "white" (the back light then goes through the LCD and the red filter and the eye perceives the field as red). Likewise, blue or green fields can be created following a similar analysis.
If a different shade for the field is desired, combinations of these three colored pixels are utilized. For example, if red and blue pixels are driven transmissive to allow light to transmit through those respective filters at uniform intensities, and the green pixels are driven "black", the field would have a magenta color, which is a mixture of blue and red. If the entire field is desired to be white, all pixels are driven "white" and light transmits through all three different colored filters across the field. The human eye averages that out as white (a combination of all primary colors). If the field is desired to be black, all LCDs are turned "black" which disallows any transmission of light through any of the colored filters (black being the absence of colors).
Still further, a combination of transmission of the same intensity through the green and red filters while blocking the blue filters would create a yellow color. By varying the intensity of either the green or the red pixels to allow more transmission between either one, the shade of yellow can be adjusted anywhere from almost red (orange) to almost green (lime green). As can therefore be understood, by varying which LCD pixels are allowed to transmit light, and at what intensity, virtually any color can be replicated, if the pixels are small enough. The human eye simply averages out the composite light output and constructs color on basis of this averaging. Because different viewing angles change the received intensity of various pixels, color rendition is changed from that intended and received when viewed straight on-axis. No adequate solution has been created for this situation.
Moreover, the off-axis angle viewing problems actually vary over the range of viewing angles that can exist for a particular LCD. In other words, in an airplane cockpit, a LCD may be tilted at 30.degree. from vertical. However, the position of the pilot's eyes also creates a situation where the top of the LCD presents a different angle than the bottom of the LCD. Therefore, contrast problems and color rendition problems may not be the same for the entire LCD, and have to be approached differently for different segments of the LCD.
It is therefore a principal object of the present invention to provide a method and apparatus for dynamically and adjustably generating liquid crystal display gray level voltages which solve or overcome the problems and deficiencies in the art.
A further object of the present invention is to provide a method and apparatus as above described which effectively and flexibly allows generation of gray level voltages.
A still further object of this present invention is to provide a method and apparatus for generating gray level voltages which can compensate for a variety of different factors associated with the performance of the LCD.
Another object of the present invention is to provide a method and apparatus as above described which has the ability to compensate for both non,time varying and the time varying factors which affect the overall performance of a LCD.
Another object of the present invention is to provide a method and apparatus as above described which allows the generation of variable gray level voltages.
A still further object of the present invention is to provide method and apparatus as above described which can compensate for off-axis viewing angles, temperature variations, and other factors unique to a particular operation and position of a LCD.
Another object of the present invention is to provide a method and apparatus as above described which can compensate for discretionary factors such as dimming or brightening of certain portions of the display.
Another object of the present invention is to provide a method and apparatus as above described which generates stable but variable gray level voltages.
A still further object of the present invention is to provide a method and apparatus as above described which is effective and reliable for a variety of different situations.
Another object of the present invention is to provide a method and apparatus for complimentary spatial modulation for off-axis LCD color and brightness control which solves or overcomes the problems and deficiencies in the art.
Another object of the present invention is to provide a method and apparatus as above described which can correct color rendition problems or brightness control problems for an LCD.
Another object of the present invention is to provide a method and apparatus as above described which can compensate for loss of color or contrast caused by viewing angle or other factors.
A still further object of the present invention is to provide method and apparatus as above described which can compensate for both non-time varying and time varying factors which affect color or brightness of the LCD.
Another object of the present invention is to provide a method and apparatus as above described which can adjust all or portions of the LCD.
A still further object of the present invention is to provide method and apparatus as above described which is economical, efficient, and durable.
These and other objects, features, and advantages of the present invention will become more apparent with reference to the accompanying specification and claims.